There is considerable interest in developing systems for accurately measuring the thickness of thin films and the optical constants of those thin films. The need is particularly acute in the semiconductor manufacturing industry where the thickness of thin film oxide layers on silicon substrates are measured. To be useful, the measurement devices must be able to determine the thickness of films with a high degree of accuracy and be able to make the measurements within very localized areas (i.e. on the micron scale).
One such device that achieves these goals has been recently introduced by the assignee herein under the trademark Optiprobe. The approach used to obtain the measurements in this device is described in detail in commonly owned U.S. Pat. No. 4,999,014, issued Mar. 12, 1991, to Gold et. al. and incorporated herein by reference.
As described in this patent, a high power, spherical, microscope objective is used to tightly focus a probe beam substantially normal to the surface of the sample in a manner to create a spread of angles of incidence. An array of discrete photodetector elements is provided to measure the intensity of individual rays within the reflected probe beam. These individual rays correspond to discrete angles of incidence with respect to the surface of the sample. Using the angularly dependent intensity measurements, the processor can evaluate the parameters of the thin film. Parameters such as layer thickness and index of refraction can be determined using the Fresnel equations.
Some of the fundamental concepts that were disclosed in U.S. Pat. No. 4,999,014 were extended to develop a high resolution ellipsometer which is described in commonly owned U.S. Pat. No. 5,042,951, issued Aug. 27, 1991, to Gold et. al. and incorporated herein by reference. In this device, the change in polarization state of the probe beam, caused by the reflection off the surface of the sample, was analyzed so that the conventional ellipsometric parameters could be used to evaluate the sample. As in the earlier cited patent, the detector arrangement in the ellipsometer included an array of discrete detector elements so that angularly dependent intensity measurements could be made.
Both of the devices as described above provide information about thin film layers. However, and as described in both patents, additional accuracy can be obtained by measuring not only the angularly dependent intensity of the rays with an array of discrete elements, but in addition, by measuring the full power of the reflected probe beam. The advantage to measuring the full power of the reflected probe beam is that it provides a much better signal to noise ratio. It should be noted that a measurement of the full power of the reflected probe beam could not be used to determine thin film thickness alone, since this signal is not linear, but varies sinusoidally with thickness. However, if the approximate thickness can be derived using the angular dependent intensity measurements, the actual thickness can be more accurately determined using the additional full power measurement.
In practice, using both angular dependent intensity detection and full probe beam power detection, accurate measurement of the thickness and optical constants of thin films down to 100 .ANG. has been achieved. Below 100 .ANG., the interference effects of the film are so small that the signal to noise ratio associated with the discrete element photodetector makes analysis by angular dependent intensity measurements difficult. In addition, the sensitivity of the standard full power measurement in the region below 100 .ANG. is also low.
Therefore, it is an object of the subject invention to provide an approach for measuring the thickness of films less than 100 .ANG. thick.
It is another object of the subject invention to provide an apparatus for determining the optical constants of films less than 100 .ANG. thick.
It is a further object of the subject invention to provide a device for evaluating parameters of a thin film with micron scale resolution.
It is a further object of the subject invention to provide a method and apparatus which can be used in conjunction with existing systems to further refine the measurement of the thickness and optical constants of thin films.